Device for protecting against accidental contact and method for protecting against accidental contact of a displaceable part

ABSTRACT

The device according to the invention has a sensor ( 5 ) for generating and detecting an electromagnetic field situated in the vicinity of a moving part ( 1 ) that should not be touched. The device according to the invention also has an evaluation unit ( 8 ) that is connected to the sensor ( 5 ) in order to evaluate sensor signals that can be generated by the sensor ( 5 ), wherein the evaluation unit ( 8 ) can generate an evaluation signal. Finally, a control unit ( 9 ) connected to the evaluation unit ( 8 ) is provided, which is designed and can be operated so as to control the movement of the part ( 1 ) as a function of the evaluation signal.

TECHNICAL FIELD

The invention relates to contact protection device and method forprotection against contact with a moving part. Both the method and thedevice can be used to protect body parts of a person from a saw blade ofa table saw.

Injuries caused by circular saws rank among the most frequent workplaceaccidents. It is important in this context to differentiate betweeninjuries in which fingers are cut or severed while feeding wood in andinjuries to fingers, hands, or arms caused when wood pieces are rapidlykicked back or become jammed.

PRIOR ART

A table saw with a safety system is known from the prior art and hasbeen disclosed by US 2002/0017184 A1. The table saw has an adjustablesaw blade, a detection system for detecting a contact between a personin the saw blade, and a braking mechanism for stopping the saw blade ifthe detection system has detected a contact between the person and thesaw blade. The system for detecting the contact between a person and asaw blade has a sensor that is embodied as a capacitive sensor. Thedisadvantage is that the proposed safety system only reacts when acontact between the person and the saw blade has already occurred. Noprevention is provided. There is therefore still a very high probabilityof injury.

Safety devices in the form of guards that cover the region above the sawblade are in fact known, but are often removed to simplify operation.They are therefore either impractical to use or do not offer sufficientprotection.

DEPICTION OF THE INVENTION

One advantage of the invention lies in the fact that the proposedcontact protection device and the method for protection against contactwith a moving part offer a sufficient degree of safety and at the sametime, do not to hinder the work of the person operating the machine.

Another advantage of the invention lies in the fact that the contactprotection device and the method for protection against contact with amoving part can be easily implemented.

It is also advantageous that a contact between the person and the partthat could injure the person, for example the saw blade, can be detectedon time so that appropriate safety measures can be taken even beforecontact with the part that could injure the person occurs.

The contact protection device according to the invention has a sensorfor generating and detecting an electromagnetic field in the vicinity ofa moving part that should not be touched. The device according to theinvention also has an evaluation unit connected to the sensor in orderto evaluate the sensor signal that can be generated by the sensor; theevaluation unit can generate an evaluation signal. Finally, theevaluation unit is connected to a control unit that is designed and canbe operated in such a way that the movement of the part can becontrolled as a function of the evaluation signal.

The method according to the invention for protection against contactwith a moving part includes the following steps. An oscillatory circuitgenerates a high-frequency electromagnetic signal. Then an evaluationunit detects and monitors a characteristic of the electromagneticsignal. Based on the characteristic, a determination is made as towhether influence should be exerted on the movement of the moving part.

Advantageous modifications of the invention ensue from the featuresdisclosed in the dependent claims.

In one embodiment of the invention, the moving part is the saw blade ofa stationary circular saw. This makes it possible to protect the personfrom injuries, for example in the form of lacerations when fingers arecut into or severed or in the form of contusions when wood pieces arerapidly kicked back or become jammed.

In a modification of the contact protection device according to theinvention, an additional sensor is provided. The two sensors are locatedon the two sides of the saw blade. This makes it possible to furtherincrease safety during the operation of the circular saw.

Alternatively, the device according to the invention can also beprovided with three additional sensors. Two of the sensors are locatedon each side of the saw blade. This makes it possible to increase theaccuracy in detecting the front edge of the saw blade.

In order to attain the object, the invention also proposes that thesensor of the device according to the invention have a flat antenna forgenerating electromagnetic waves. This simplifies implementation.

In another embodiment of the invention, the sensor is located on theunderside of a jam guard for the saw blade. This has the advantage ofprotecting the sensor from mechanical damage and at the same time,allowing for design freedom with regard to the top side of the workingsurface of the circular saw.

The contact protection device according to the invention isadvantageously provided with an oscillatory circuit connected to theantenna and the evaluation unit is designed so that it can evaluate thedetuning of the oscillatory circuit.

It is particularly advantageous if the waves emitted by the antenna ofthe device according to the invention lie in the range of the ISM band.Using the frequencies within this band is permitted worldwide andsignificantly facilitates the process of obtaining permits from theinvolved authorities.

In a modification of the method according to the invention forprotection against contact with a moving part, the detuning of theoscillatory circuit serves as a characteristic. This makes it easilypossible to differentiate between whether the work piece or a finger ofthe person guiding the work piece is about to come into contact with thesaw blade.

In another modification of the method according to the invention, theresonance frequency of the oscillatory circuit and/or the speed of thechange in the resonance frequency and/or the number of resonancefrequencies can serve as a characteristics. These characteristics areeasy to determine and permit a sufficient conclusion to be drawnregarding the situation at hand.

Finally, with the method according to the invention, a change in thetorque of the moving part can be detected and the change in the torquecan additionally be used to determine whether influence should beexerted on the movement of the moving part. This further increases theaccuracy of the detection.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of exemplary embodiments of the invention will be explained indetail below in conjunction with twelve figures.

FIG. 1 shows a schematically depicted detail of a circular saw with adevice according to the invention.

FIG. 2 is a block circuit diagram of a possible embodiment form of theelectrical part of the device according to the invention.

FIG. 3 shows a detail of the circular saw with the saw blade, butwithout a work piece.

FIG. 4 shows the frequency curve of the measurement signal when thecircular saw is running, without a work piece.

FIG. 5 shows the circular saw when it is sawing a work piece apart.

FIG. 6 shows the frequency curve of the measurement signal when thecircular saw is cutting through the work piece.

FIG. 7 shows the circular saw when it is cutting through a work pieceand a finger at the same time.

FIG. 8 shows the frequency curve of the measurement signal when thecircular saw is cutting into a work piece and a finger.

FIG. 9 is a top view of a possible placement of the sensor in relationto the circular saw blade.

FIG. 10 shows a flow chart of a possible embodiment of the contactprotection method according to the invention.

FIG. 11 shows a table saw with two sensors for contact detection.

FIG. 12 shows a table saw with four sensors for contact detection.

FIG. 13 shows a measured frequency curve when cutting through wood and afinger.

WAYS TO EMBODY THE INVENTION

FIG. 1 shows a detail of a stationary circular saw with the contactprotection device according to the invention. A work piece 2, forexample a piece of wood, is slid in the movement direction 7 across atable, parallel to the saw blade 1. The saw blade 1 therefore cuts thework piece 2 apart at the cutting edge 31. In order to guide the workpiece 2 and to hold it down, a person holds the work piece with hisfingers in the immediate vicinity of the saw blade 1. This is indicatedin FIG. 1 by a schematically depicted finger labeled with the referencenumeral 3. In order to prevent the finger of the person from coming intocontact with the saw blade 1, the contact protection device according tothe invention is provided with a sensor 5 located in the immediatevicinity of the saw blade 1. In the exemplary embodiment shown in FIG.1, the sensor 5 is located on the underside 4.1 of the jam guard 4 thatencompasses the saw blade 1. The jam guard 4 can be embodied as aplastic part and prevents the saw blade 1 from jamming, for example whenit is set at an angle in relation to the work table or when the workpiece exerts a load on the saw blade lateral to the sawing direction. Inaddition, the jam guard 4 allows different thicknesses of saw blades tobe used without causing the saw blade 1 to jam. The jam guard 4 is alsoreferred to below as the radome (radar dome). The jam guard 4 serves toprotect the sensor 5 from damage and, if it is made of plastic,simultaneously serves to electrically insulate the sensor 5.

The antenna of the sensor 5 is embodied as a patch antenna and has aplanar design. The antenna is excited with a frequency of 2.45 MHz, forexample, in relation to a mass 6 disposed underneath it, which is alsoreferred to as a reference potential.

If the saw blade 1 is running at idle as shown in FIG. 3, i.e. neither awork piece 2 nor a finger 3 or hand of a person is in the vicinity ofthe saw blade 1, then this yields the frequency spectrum shown in FIG.4. It is clear that the frequency spectrum 32 has only one resonancefrequency fr at approx. 2.48 GHz. The bottom region of FIG. 4 shows thecorresponding Smith chart for the operation at idle.

If a work piece 2 is then brought into the vicinity of the saw blade 1,as shown in FIG. 5, then this yields the frequency graph 33 shown at thetop in FIG. 6; it is clear that the reference frequency fr has shiftedslightly and is now at approx. 2.44 GHz. It is also clear from FIG. 6that the adaptation has changed slightly. However, the curve is stillsmooth. The change occurs in the range from 2 to 10 cm per second andcan be adaptively tracked as a measurement shift.

If a finger 3 then approaches the saw blade 1 as shown in FIG. 7, thenthis results in a severe detuning of the oscillatory circuit, which isevidenced by a shifting of the resonance frequency fr and by thepresence of a number of subresonances. FIG. 8 shows the correspondingfrequency graph with the associated frequency curve 34. Now the curveshows a series of resonance frequencies fr1, fr2, fr3, and fr4. Thecorresponding Smith chart is shown in the bottom region of FIG. 8.

It is clear that as the finger 3 approaches the saw blade 1, theresonance frequency is continuously shifted downward. In addition,dielectric losses cause a broadening of the resonance curve. This canalso be seen in FIG. 13. In the graph shown in FIG. 13, the damping indB is plotted vertically and the frequency is plotted as increasingtoward the right. The frequency curves FV1 toward the front representthe leading edge of the wood in contact with the saw blade 1 and thefrequency curves FV2 further toward the back represent the finger 3 incontact with the saw blade 1.

This detuning can occur at the advancing speed at which the work pieceis slid along the saw blade 1, which would lead to a cutting or severingof the finger 3. With the aid of an adaptive algorithm for adaptivefrequency regulation, it is possible to detect additional subordinatemaxima or additional resonance frequencies fr1–fr4, which makes itpossible to detect the presence of a finger or hand.

However, the detuning can also occur with an abrupt movement of thefinger 3, caused by a kickback or jamming of the work piece 2. A rapidchange in the frequency spectrum can be evaluated directly and permits aconclusion to be drawn as to whether the work piece has caused akickback or the saw blade 1 has become jammed or whether there is infact no danger in this regard.

Once the saw blade has reached its nominal speed, the Doppler effectgenerated by the saw blade 1 yields a constant value and can thereforebe eliminated so that the Doppler effect does not cause falsetriggerings.

If other data are available, for example the slippage of the saw and/orthe change in the torque of the saw blade 1, then these can be includedin the evaluation.

The parameters of the antenna and the material used to produce the jamguard 4 are selected so that normal wood demonstrates an optimummeasurement effect, i.e. a favorable adaptation at the center frequency.

FIG. 9 shows a top view of the location of the sensor with the antenna12, a capacitor 14 connected to it, an inductance 15, and a varactordiode 13 connected to ground. It is clear that the antenna 12 is locatedin the vicinity of the saw blade 1, parallel to the surface of the worktable 11. In the exemplary embodiment shown in FIG. 9, the antenna 12 isalso located at the center of the saw blade 1 when viewed from above.

If other sensors are provided, then they can be positioned as shown inFIGS. 11 and 12.

In FIG. 11, a first sensor 5.1 is located on the right side of the sawblade 1 and a second sensor 5.2 is located on the left to the saw blade.The two sensors are positioned in the center of the saw blade 1 whenviewed from above.

In the embodiment shown in FIG. 12, two additional sensors 5.3 and 5.4are positioned in the front region of the saw blade 1; the sensor 5.3 islocated along with the sensor 5.1 on the one side of the saw blade andthe sensor 5.4 is located along with the sensor 5.2 on the other side ofthe saw blade. The two sensors 5.3 and 5.4 are positioned in the worktable 11 so that they can detect when a body part, for example a fingeror hand, moves into the vicinity of the saw edge. The two sensors 5.1and 5.2 can detect kickbacks caused by the work piece 2.

For improved position determination, it is helpful to work with two ormore sensors. In order to increase the accuracy of detection at the sawedge 31, it is possible to provide two additional sensors, as shown inFIG. 12. The individual antennas 12 here can also be rotated out fromthe plane by a certain angle, for example by 30°, to permit bettercoverage and detection of the front region of the circular saw, i.e. thesaw edge 31. The jam guard 4 here can still be embodied as planar on thetop side since the sensors are positioned on the underside 4.1 of thejam guard 4.

If several sensors 5.1, 5.2, and 5.3 are used, then the block circuitdiagram depicted in FIG. 2 is used for the interconnection of theelectrical components of the contact protection device according to theinvention. The sensors 5.1, 5.2, and 5.3 are connected to an evaluationunit 8, which evaluates the measurement signals emitted by the sensors5.1, 5.2, and 5.3 and sends a corresponding selection signal to acontrol unit 9. Based on the selection signal, the control unit 9 thendetermines whether a motor 10 that serves to drive the saw blade 1should be influenced, for example by means of braking. The control unit9 can also be connected to other components, for example an active brakeor a device for automatically lowering the saw blade 1. In addition tothe sensors 5.1, 5.2, and 5.3, still other sensors can also be connectedto the evaluation unit 8.

The reflection damping can be evaluated by means of a standing wavebridge or a directional coupler. A phase change can be detected with theaid of a phase detector. To this end, the operating frequency is sweptin the permissible range. It is also possible to use measuring methodsof the kind used in network analyzers (vector network analyzers).

In order to stop the circular saw or the saw blade 1 of the circular sawwhen the saw blade is contacted or just before this occurs, a method isused that is based on the resonance shift in the microwave rangegenerated by the influence of body parts, for example a finger or hand.The ISM band from 2,400 MHz to 2,483.5 MHz is selected as the frequencyband since antennas for this frequency band are readily available andthe main material to be machined, namely wood or plastic, is stillpenetrated to a material depth of approximately 10 cm due to the skineffect. Alternative frequency ranges are 433 MHz, 866 MHz, or 5.8 GHz.The use of these ISM bands assures the possibility for worldwideapproval.

The fundamental concept of the invention is based not on a radardistance measurement but instead, makes an evaluation in the microsecondrange of a change in the dielectric encompassing the antenna. It is alsopossible to evaluate an electric change.

If a finger, whose dielectric constant εr lies in a range between 40 and80, approaches the saw blade, then a severe detuning of the antennaoccurs.

In order to limit the required frequency band, an adaptive (slow)regulation can be used, which masks material changes, for exampledifferent material thicknesses, a leading and trailing edge of the workpiece, knots, and the like, and always shifts the operating frequencyinto an optimum position.

This makes it possible to also detect external interference from otherISM devices such as microwave ovens, devices with a Bluetooth interface,or small radio networks for data transmission and to effectivelysuppress this interference through suitable measures.

A slow adaptive regulation can be used to mask material changes and toalways shift the operating frequency into an optimum position. To thisend, the center frequency of the antenna 12 is detuned via theoscillatory circuit comprised of the capacitor 14, the varactor diode13, and the inductance 15. The array here can either be connected onceor in the symmetrical center in terms of the resonant length. Ifexternal interferences at certain frequencies are detected, then theevaluation frequency can also be shifted.

FIG. 10 shows the flowchart for the evaluation of the sensor signal. Thecontact prevention method is described in conjunction with a contactprevention device that has two sensors. If the first sensor 5.1 signalsa resonance shift, represented by step 21 and the arrow labeled Jleading from it, then in steps 22 and 23, a determination is made withthe aid of the second sensor 5.2 as to whether the measurement signalsemitted by the two sensors indicate similar effects. If this is not thecase, i.e. if the two measurement signals differ significantly from eachother, then it can be assumed that the finger or fingers are nottouching the saw blade and will be conveyed past it. The process thenfollows branch 24 to step 25. But if the two sensors 5.1 and 5.2 emitsimilar and rapidly changing measurement results, then it can be assumedthat the work piece has caused a kickback, requiring an immediate stop28 of the saw blade 1. The detection is executed by means of a fastalgorithm 27. In the event of a slow change, characterized by step 29, aslow algorithm 30 is used to determine whether the change has beencaused by the work piece or whether a body part has traveled into theimmediate vicinity of the saw blade 1. The slow algorithm 30 detectsthis and in the event of danger, the process moves to step 28 in whichmeasures are taken to stop the saw blade. Otherwise, the process movesto step 25.

Since the movements that occur are usually steady, it is also possibleuse SAR (synthetic aperture radar) methods.

The device according to the invention is not limited to the use on acircular saw. The device according to the invention can also be used toprotect human body parts in the vicinity of band saws, presses, drills,or special purpose machines with protection devices. In order toaccommodate the sensor, it is sufficient to provide a flat surfaceapproximately 50 cm by 50 cm, which is covered by the dielectric radome.The sensor can be supplied with voltage via electrical connections andthe measurement data and evaluation data can be transmitted via a datainterface.

The embodiment according to the invention has the advantage over anoptical monitoring of not requiring an optical connection between thefingers and the sensors. Instead, objects such as the work piece can bepositioned between the sensor and the fingers.

The preceding description of exemplary embodiments according to thecurrent invention is intended exclusively for illustrative purposes andis not intended to limit the invention. Various changes andmodifications can be made without going beyond the scope of theinvention and its equivalents.

1. A device for protecting a body part of a person from contact with arotating saw blade of a circular saw, comprising a sensor for generatingand detecting an electromagnetic field situated in the vicinity of thesaw blade (1); an evaluation unit (8) connected to the sensor (5) forevaluation of sensor signals that are generated by the sensor (5), andgeneration of an evaluation signal; and a control unit (9) connected tothe evaluation unit (8), which is configured so as to control a movementof the saw blade (1) as a function of the evaluation signal, wherein thesensor (5) is positioned on an underside (4.1) of a jam guard (4) forthe saw blade (1).
 2. The device according to claim 1, characterized inthat another sensor (5.2) is provided, wherein the two sensors (5.1,5.2) are positioned on the two sides of the saw blade (1).
 3. The deviceaccording to claim 1, characterized in that three additional sensors(5.2, 5.3, 5.4) are provided, wherein two sensors (5.1, 5.3; 5.2; 5.4)are positioned on each side of the saw blade (1).
 4. The deviceaccording to claim 1, characterized in that the sensor (5) has a flatantenna (12) for generating electromagnetic waves.
 5. The deviceaccording to claim 4, characterized in that an oscillatory circuitconnected to the antenna (12) is provided and that the evaluation unit(8) is designed so that it can evaluate the detuning of the oscillatorycircuit.
 6. The device according to claim 4, characterized in that thewaves that can be emitted by the antenna (12) lie in the range of theISM band.
 7. A method for protecting a body part of a person fromcontact with a rotating saw blade of a circular saw, comprising thesteps of generating a high-frequency electromagnetic signal by anoscillatory circuit and an antenna (12) of a sensor (5) positioned on anunderside (4.1) of a jam guard (4) for the saw blade (1); detecting andmonitoring a characteristic of the electromagnetic signal by anevaluation unit; and based on the characteristic, making a determinationas to whether influence should be exerted on the rotation of the sawblade (1).
 8. The method according to claim 7, characterized in that thedetuning of the oscillatory circuit serves as the characteristic.
 9. Themethod according to claim 7, characterized in that the resonancefrequency (fr) of the oscillatory circuit and/or the speed of the changein the resonance frequency and/or the resonance broadening serves as thecharacteristic.
 10. A method for protection against contact with amoving part, characterized by means of the following steps: ahigh-frequency electromagnetic signal is generated by an oscillatorycircuit and an antenna (12), an evaluation unit (8) detects and monitorsa characteristic of the electromagnetic signal, and based on thecharacteristic, a determination is made as to whether influence shouldbe exerted on the movement of the moving part (1), and characterized inthat a change in the torque of the moving part (1) is detected and thechange is also used to determine whether influence should be exerted onthe movement of the moving part (1).